EFFECT OF NITROGEN-FERTILIZER TIMING AND RATE ON INORGANIC NITROGEN STATUS, FRUIT COMPOSITION, AND YIELD OF GRAPEVINES

A four-year (1987-90) trial was conducted to study the effects of timi ng and rate of N fertilizer application on furrow-irrigated Barbera, G renache, French Colombard, and Chenin blanc grapevines. The objectives were to further determine optimum N fertilizer practices for importan t wine cultivars in the San Joaquin Valley and to compare inorganic N constituents, NO3-N and NH4+-N, for N status determination. The six tr eatments included four N fertilizer application timings at budbreak (B B), fruit set (FS), veraison (V), and post-harvest (PH) with a 56 kg N /ha rate, as well as BB with a 2X rate of 112 kg N/ha and control (CO) . Vine N status was determined by NO3--N and NH4+-N levels and their s um, total inorganic N (TN), in bloom and veraison petioles. Yield and fruit composition parameters were measured for vine response. Cultivar responses varied widely, with Grenache and Barbera being the high and low extreme cultivars, respectively, in petiole TN status and respons iveness to N treatment; French Colombard and Chenin blanc were interme diate. Generally, TN status increased with increasing N rate (CO BB56 BB112). Nitrogen fertilizer timing demonstrated that the more recent N applications often resulted in a higher petiole TN status at the next petiole sampling; i.e., PH56 and BB56 treatment petioles had higher T N status at bloom while FS56 was higher at veraison. Post-harvest timi ng was found to be particularly effective in supplying petiole TN in t he spring; i.e., PH56 sometimes resulted in bloom petiole TN levels co mparable to BB112 in all four cultivars and higher than BB56 in Grenac he, French Colombard, and Chenin blanc. Petiole NO3--N levels were mor e sensitive than NH4+-N levels to fertilizer treatment. The greater co ntribution of NO3--N to TN and their high correlation coefficients as compared to NH4+-N and TN would place in question the utilization of N H4+-N as an additional measurement of N status. Grape soluble solids w as the most responsive fruit parameter to N treatment. Generally, N fe rtilizer decreased soluble solids over CO, irrespective of timing, and with the highest rate (112 kg N/ha) resulting in the largest decrease . Grape yields were only affected in Grenache, with all of the N treat ments increasing yields over CO except V56. This result, plus other ne gative effects of the V56 treatment, suggest that veraison was the lea st desirable timing of those compared. Otherwise, timing effects on fr uit parameters were minor.